1. Field of the Invention
The present invention relates to improvement of a reverse shift device in a transmission for an automobile or the like, the reverse shift device having a reverse gear train composed of a driving gear, a driven gear, and an idle gear which moves in the axial direction and is thereby engaged with or disengaged from the two gears.
2. Description of Related Art
Such a type of reverse shift device in a transmission is disclosed as a conventional technique in Japanese Patent Application No. 2010-054942 (paragraphs [0002] to [0011]; FIGS. 1 and 7 to 9) filed by the applicant of the present invention. As shown in FIGS. 7 to 10, a transmission casing 10 of the transmission includes first and second casings 11 and 13 and a shift bracket 12 liquid-tightly intervening therebetween, and the three members 11 to 13 are united together with bolts, whereby the interior of the transmission casing 10 is divided into a first chamber S1 on a side toward the first casing 11 and a second chamber S2 on a side toward the second casing 13. In this transmission, a counter shaft 16 and an output shaft 17 are disposed in parallel with each other and are supported by the transmission casing 10 via bearings in such a manner as to be rotatable within the transmission casing 10; an input shaft 15 is rotatably driven by an engine via a clutch and is coaxially and rotatably coupled with one end of the output shaft 17 via a needle bearing; and the other end of the output shaft 17 is connected to driving wheels. Rotation of the input shaft 15 is transmitted at all times to the counter shaft 16 via a transmission gear pair 18, and there are provided, between the counter shaft 16 and the output shaft 17, a plurality of speed change gear pairs 20 to 23 within the first chamber S1 for selective power transmission and a reverse gear train 24 within the second chamber S2.
As shown mainly in FIG. 7, driving gears of the 1st-gear and 2nd-gear pairs 20 and 21 are fixed to the counter shaft 16, and the respective driven gears are rotatably supported on the output shaft 17. A first changeover clutch mechanism 25 provided between the two driven gears is a well-known synchronous engaging mechanism and has a clutch hub 25a which rotates with the output shaft 17, and a sleeve 25b which is spline-engaged with the clutch hub 25a at its outer circumference in an axially slidable manner. In a state in which the sleeve 25b is at the illustrated neutral position, the driven gears on the opposite sides of the sleeve 25b are not connected to the output shaft 17 and are thus freely rotatable; when a first shift fork 40, which partially constitutes a shift mechanism 29 (see FIG. 10) to be described later, moves the sleeve 25b to a side toward the 1st-gear pair 20, the driven gear of the 1st-gear pair 20 is connected to the output shaft 17, and power transmission is performed through the 1st-gear pair 20; and when the sleeve 25b is moved to a side toward the 2nd-gear pair 21, power transmission is performed through the 2nd-gear pair 21. The driving gear of the 3rd-gear pair 22 is fixed to the counter shaft 16, and the driven gear of the 3rd-gear pair 22 is rotatably supported on the output shaft 17 at its end portion toward the input shaft 15. A second changeover clutch mechanism 26 is a synchronous engaging mechanism similar to that of the first changeover clutch mechanism 25 and is provided between the driven gear of the 3rd-gear pair 22 and the driving gear of the transmission gear pair 18; when a second shift fork 41 moves a sleeve 26b to a side toward the 3rd-gear pair 22, the driven gear of the 3rd-gear pair 22 is connected to the output shaft 17, and power transmission is performed through the 3rd-gear pair 22; and when the sleeve 26b is moved to a side toward the transmission gear pair 18, the driving gear of the transmission gear pair 18 is connected to the output shaft 17, and the input shaft 15 and the output shaft 17 are thereby connected directly, whereby power transmission is performed in 4th gear.
A driving gear 23a of the 5th-gear pair 23 is rotatably supported on the counter shaft 16; a driven gear 23b of the 5th-gear pair 23 is fixed to the output shaft 17; and a third changeover clutch mechanism 27 which engages the driving gear 23a to and disengages the driving gear 23a from the counter shaft 16 is a so-called lever-type synchronous engaging mechanism and includes the following members: a clutch hub 27a which is formed coaxially and integrally with the driving gear 23a and has external splines 27a1 formed at its outer circumference and two cutouts 27a2 formed at circumferential positions; a sleeve 27b having internal splines 27b2 which are formed at its inner circumference and are engaged with the external splines 27a1 in an axially slidable manner; a rotating member 27c fixed to a distal end of the counter shaft 16 and having external splines 27c1 which are formed at its outer circumference and are engaged with the internal splines 27b2 in an axially slidable manner; a synchronizer ring 27d which is provided between the rotating member 27c and the clutch hub 27a and forms a cone friction clutch in cooperation with the rotating member 27c; and two semicircular lever members 27e which press the synchronizer ring 27d against the rotating member 27c at an initial stage of movement of the sleeve 27b in either axial direction and thereby synchronize the driving gear 23a and the rotating member 27c (whereby the counter shaft 16 and the output shaft 17 are synchronized at a predetermined change gear ratio between the driving and driven gears 23a and 23b). The sleeve 27b has cutout grooves 27b3 having a trapezoidal cross section and formed along its inner circumference at the axial centers of the internal splines 27b2. When the sleeve 27b is at the neutral position which is shown in FIG. 7 at an upper half of the third changeover clutch mechanism 27, the ends of central portions of the lever members 27e urged outwardly by springs (not shown) are elastically pressed against the bottom surfaces of the cutout grooves 27b3.
When a third shift fork 42, which partially constitutes the shift mechanism 29 to be described later, moves the sleeve 27b of the third changeover clutch mechanism 27 to a side toward the rotating member 27c (rightward in FIG. 7), first, the internal splines 27b2 of the sleeve 27b are engaged with external splines 27d1 of the synchronizer ring 27d; next, the distal end portions of the lever members 27e hit against a base portion of an inclined left side surface of the cutout groove 27b3 and move rightward, and, thus, the lever members 27e pivot on one side 27e 1 of their proximal end portions in contact with the end surface of the clutch hub 27a (see FIG. 8); accordingly, the distal end portions come into contact with the synchronizer ring 27d and press the synchronizer ring 27d rightward. Thus, the clutch hub 27a and the rotating member 27c are synchronized in rotation; subsequently, the clutch hub 27a moves to a 5th-gear shift position which is shown at a lower half of the third changeover clutch mechanism 27 (see FIG. 7); and the internal splines 27b2 are engaged with the external splines 27c1 of the rotating member 27c, and the driving gear 23a of the 5th-gear pair 23 is connected to the counter shaft 16, whereby power transmission is performed through the 5th-gear pair 23. In this condition, the lever members 27e are moved inward against the aforementioned springs (not shown) and onto crests of the internal splines 27b2 of the sleeve 27b. 
As shown mainly in FIGS. 7 and 9, the reverse gear train 24 is composed of a driving gear 24a formed integral with the counter shaft 16; a driven gear 24c which is formed integral with the sleeve 25b of the first changeover clutch mechanism 25 at the outer circumference of the sleeve 25b and rotates at all times with the output shaft 17; and an idle gear 24b supported rotatably and axially movably by a support shaft 24d which is provided on the transmission casing 10 in parallel with the counter shaft 16 and with the output shaft 17. The idle gear 24b has an annular groove 24b1 formed coaxially at its outer circumference. A reverse arm 43 is disposed within a lower region of the transmission casing 10 and extends in a direction substantially orthogonal to the support shaft 24d; a proximal end portion of the reverse arm 43 is pivotally supported to the transmission casing 10 through a pivot pin 44 (see FIG. 9) orthogonal to the longitudinal direction of the reverse arm 43; and a pin 43a formed at a distal end portion of the reverse arm 43 is engaged with the annular groove 24b 1 of the idle gear 24b. When a reverse fork 34, which partially constitutes the shift mechanism 29 to be described next, pivots the reverse arm 43 about the pivot pin 44, the idle gear 24b is moved in the axial direction along the support shaft 24d between a disengagement position and an engagement position; the idle gear 24b at the disengagement position is represented by the solid line and is disengaged from both of the driving gear 24a and the driven gear 24c; and the idle gear 24b at the engagement position is represented by the dash-dot-dot line and is engaged with the driving gear 24a and with the driven gear 24c, whereby power transmission is performed through the reverse gear train 24.
Next, the shift mechanism 29 will be described with reference to FIG. 10. Three fork shafts 30 to 32 of the shift mechanism 29 have shift pieces 30a to 32a provided unitarily at their respective one ends and have stop rings 30c to 32c latched respectively thereto at positions located at the same distance from the end surfaces of the shift pieces 30a to 32a. Distal end portions of the fork shafts 30 to 32 are axially slidably fitted into support holes 11a to 11c, respectively, formed in the first casing 11 of the transmission casing 10 at the same pitch on the same plane, whereas portions of the first through third fork shafts 30 to 32, respectively, between the shift pieces 30a to 32a and the stop rings 30c to 32c are axially slidably fitted into holding holes 12a to 12c, respectively, formed in the shift bracket 12 of the transmission casing 10 at the same pitch on the same plane. Thus, the fork shafts 30 to 32 disposed in parallel with each other can move in the axial direction between a position where the end surfaces of the shift pieces 30a to 32a come into contact with one side surface of the shift bracket 12 and a position where the stop rings 30c to 32c come into contact with the other side surface of the shift bracket 12. Also, a holding hole 12d for holding a fixed shaft 33 is formed in the shift bracket 12 in parallel with and adjacent to the holding hole 12c on the same plane as that where the holding holes 12a to 12c are formed.
The shift bracket 12 has a guide hole 12e formed therein in such a manner as to orthogonally intersect the holding holes 12a to 12c, pass through their centers, and reach the holding hole 12d. Three notches 30d are formed at certain axial intervals in an upper circumferential portion of the first fork shaft 30 between the stop ring 30c and the end surface of the first shaft piece 30a. A detent device is formed by means of the notches 30d, a ball 36 provided in the guide hole 12e, and a spring 36a which is provided within the guide hole 12e between the ball 36 and a plug 36b for closing the inlet of the guide hole 12e and which presses the ball 36 toward the notches 30d. The detent device elastically locks the first fork shaft 30 at a neutral position and at two shift positions located on axially opposite sides, respectively, of the neutral position. Although unillustrated, a similar detent device is provided for each of the second and third fork shafts 31 and 32. An interlock mechanism for preventing simultaneous shift of the fork shafts 30 to 32 is formed by means of: two interlock pins 38 provided axially slidably in a portion of the guide hole 12e between the holding hole 12a and the holding hole 12c; a plurality of recesses (reference numeral 30e denotes one of the recesses) formed in outer circumferential surfaces of the first through third fork shafts 30 to 32; and an interlocking pin 38a provided in a hole which extends through the second fork shaft 31 at the position of the recess. The first and second shift forks 40 and 41 for moving the sleeves 25b and 26b of the first and second changeover clutch mechanisms 25 and 26 as mentioned above are fixed to intermediate portions of the first and second fork shafts 30 and 31. The third shift fork 42 for moving the sleeve 27b of the third changeover clutch mechanism 27 as mentioned above is formed at an end of the third shift piece 32a of the third fork shaft 32.
As shown in FIGS. 9 and 10, the reverse fork 34 for operating the reverse arm 43 is assembled into the shift mechanism 29 as follows: the third fork shaft 32 and the fixed shaft 33 are slidably inserted into a pair of the guide holes 34d and 34e, respectively, formed in parallel with each other. The reverse fork 34 can move between the illustrated neutral position and a reverse shift position; in a state in which the third fork shaft 32 is at the illustrated neutral position, the reverse fork 34 is at the neutral position such that one end surface of the reverse fork 34 is in contact with the stop ring 32c latched to the third fork shaft 32; and the reverse fork 34 is at the reverse shift position such that the other end surface of the reverse fork 34 is in contact with a stop ring 33a latched to the fixed shaft 33. A driving pin 34c projects from a head 34h provided at the distal end of an arm 34b (see FIG. 9) extending downward from the reverse fork 34; the driving pin 34c is slidably inserted between two flat inner side surfaces of an elongated hole 43b formed in a longitudinally intermediate portion of the reverse arm 43 with substantially no gap between the driving pin 34c and the inner side surfaces; and a stop ring 34g and a washer are used to prevent detachment of the driving pin 34c from the elongated hole 43b. Thus, the reverse arm 43 pivots about the pivot pin 44 in interlocking relation with movement of the reverse fork 34, whereby the idle gear 24b moves between the disengagement position and the engagement position.
As shown in FIG. 10, the reverse fork 34 has a closed-bottomed guide hole 34f formed therein and extending radially from the guide hole 34e, and the fixed shaft 33 has two notches 33b formed on its outer circumferential surface. A detent device is formed by means of the two notches 33b, a ball 37 provided within the guide hole 34f, and a spring 37a for pressing the ball 37 toward the notches 33b. The detent device elastically locks the reverse fork 34 at the neutral position and at the reverse shift position. Furthermore, the guide holes 34d and 34e of the reverse fork 34 communicate with each other through a hole 34a whose diameter is greater than the minimum wall thickness between the guide holes 34d and 34e; recesses 32d and 33c are formed on the outer circumferential surfaces of the third fork shaft 32 and the fixed shaft 33, respectively, at the respective positions corresponding to the hole 34a; and the hole 34a accommodates therein a ball 35 which alternately engages with the recess 32d and the recess 33c. Through employment of such a configuration, when the third fork shaft 32 is moved leftward from the illustrated neutral position, the reverse fork 34 is pressed by the stop ring 32c and thereby moves with the third fork shaft 32; when the third fork shaft 32 reaches the reverse shift position, the idle gear 24b is moved to the engagement position through the driving pin 34c and the reverse arm 43, whereby power transmission is performed through the reverse gear train 24; and when the third fork shaft 32 is returned to the neutral position, the reverse fork 34 also returns to its initial position. However, when the third fork shaft 32 is moved rightward from the illustrated neutral position, the detent device holds the reverse fork 34 at its position, and, since the ball 35 is engaged with the recess 33c, the reverse fork 34 remains at a halt; i.e., the reverse fork 34 is not moved.
In a state in which the fork shafts 30 to 32 of the shift mechanism 29 are at their neutral positions, radial end portions of the shift pieces 30a to 32a provided on the fork shafts 30 to 32, and squarish-letter-C-shaped cutouts (in FIG. 10, only a cutout 30b of the first shift piece 30a is shown, and other cutouts are behind other members) in the radial end portions are aligned in a direction orthogonal to the fork shafts 30 to 32. In this state, a selecting operation on an automatic or manual gear shifter (not shown) pivotally moves a shift and select shaft (not shown), and a distal end portion of an operating arm (not shown) provided at an end of the shift and select shaft is thereby selectively engaged with the cutout of any one of the shift pieces 30a to 32a; then, a shift operation on the gear shifter moves the shift and select shaft axially rightward or leftward, whereby the fork shaft 30, 31, or 32 selected by the operating arm moves rightward or leftward.
When the third fork shaft 32 is selected by the operating arm of the shift and select shaft and is shifted rightward (toward a 5th-gear side) in FIGS. 7 and 10, as mentioned above, first, the synchronizer ring 27d is engaged with the rotating member 27c, whereby the clutch hub 27a and the rotating member 27c are synchronized in rotation; next, the internal splines 27b2 of the sleeve 27b are engaged with the external splines 27c1 of the rotating member 27c, whereby the driving gear 23a of the 5th-gear pair 23 is connected to the counter shaft 16 (see the lower half of the third changeover clutch mechanism 27 in FIG. 7), and power transmission is performed through the 5th-gear pair 23. Even when the third fork shaft 32 is shifted rightward in this manner, as mentioned above, the reverse fork 34 remains at a halt at the neutral position (see FIG. 10).
Similarly, when the third fork shaft 32 is selected and is shifted leftward (toward a reverse side) in FIGS. 7 and 10, as mentioned above, first, the synchronizer ring 27d is engaged with the rotating member 27c, whereby the counter shaft 16 and the output shaft 17 are synchronized at a predetermined change gear ratio; however, since, in shift to reverse, the output shaft 17 connected to the driving wheels is substantially at a halt, the counter shaft 16 is also substantially at a halt. The reverse fork 34 is pressed by the stop ring 32c and moves with the third fork shaft 32; the reverse arm 43 with the driving pin 34c of the reverse fork 34 inserted without any gaps between opposed inner side surfaces of its elongated hole 43b pivots about the pivot pin 44 provided at its proximal end and thereby moves the idle gear 24b through a pin 43a provided at its distal end; and when the third fork shaft 32 and the reverse fork 34 reach the reverse shift position, the reverse arm 43 and the idle gear 24b reach the engagement position, whereby the idle gear 24b is engaged with the driving gear 24a and with the driven gear 24c. 
Although the driving gear 24a and the driven gear 24c are synchronized as mentioned above, since the driving gear 24a and the driven gear 24c are provided on the counter shaft 16 and the output shaft 17, respectively, which are substantially at a halt, the above-mentioned engagement does not raise gear rattle. Also, since the reverse fork 34, the reverse arm 43, and the idle gear 24b are shifted in association with the leftward (toward the reverse side) shift of the third fork shaft 32 from the neutral position, but are not shifted in association with the rightward (toward the 5th-gear side) shift, the amounts of movements of these members 34, 43, and 24b reduce accordingly, whereby these members can avoid interfering with peripheral members, such as the shift bracket 12.